A wide range of molecules have been employed for delivering polynucleotides and other active agents to cells. For example, polymers such as polyethylenimine or poly(beta-aminoesters) have been used to effectively complex DNA for delivery into cells. Polymers in these classes of delivery agent typically contain amine functionalities that serve to electrostatically bind to DNA to form nanoparticles that are then taken up by the cell via endocytosis. Once in the cell, these amine groups serve to buffer the endosome and cause an influx of ions due to the proton-sponge mechanism. The resulting burst of the endocytic vesicle leads to the release of the payload of the particle, which is then free to travel to the nucleus where the DNA is expressed.
While such polymer based systems have been used extensively for DNA delivery, the delivery of other molecules, such as RNA, presents distinct challenges. In many cases, polymeric materials do not work as effectively for RNA delivery. This is likely due to differences in the chemical structure of the RNA being delivered compared to DNA. RNA are generally short, linear fragments containing additional hydroxyl moieties on each ribose ring. These differences necessitate an alternative approach that is suited for complexation with short RNA strands. In particular, an improved delivery system is required for the use of siRNA for agricultural and pharmaceutical applications. The delivery system needs to protect siRNA from nuclease degradation, allow for the proper concentration and distribution profile in the target tissues, facilitate efficient uptake of siRNA into target cells, and release siRNA into cytoplasm to knockout expression of the target gene.
Promising results have been achieved with materials that form liposomes or lipoplexes that entrap the RNA or form nanoparticles, which can then be internalized by a cell. The materials utilized to form a lipid-based delivery system generally consist of a positively charged headgroup and a hydrophobic tail. The charged portion serves to electrostatically bind the negatively charged RNA, while the hydrophobic tail leads to self-assembly into lipophilic particles. Such cationic lipids are promising but still fall short of the transfection efficiency achieved by viral vectors. Few advances have been made in the field, in part due to the limited structural diversity of these lipid-like molecules, which is a result of the difficult synthetic procedures oftentimes required to access these structures.
Thus, there exists a continuing need for lipidoid molecules that possess an improved ability to deliver RNA, as well as other active agents, to cells over existing amine-containing lipidoid materials.